1. Technical Field
The present invention relates generally to digital data transmission and in particular to a method in apparatus for recovering a clock signal. Still more particularly, the present invention relates to a phase locked loop circuit in which a reference clock is absent.
2. Description of the Related Art
Services such as voice mail, video on demand, and voice activated accessing of information from remote text and video libraries across the country are being made available over the internet and through other sources through various broad band technologies. These services are made available through components such as synchronous optical network (SONET), a synchronous transfer mode (ATM) and intelligent networks (IN) that are found in broad band communications systems. Other services include computer serial input/output (I/O) data channels such as Fibre Channel and serial storage architecture (SSA). These services are provided through a communications network containing a system of interconnected facilities designed to carry traffic from a variety of telecommunications sources. A "network" includes nodes and links. "Nodes" are typically switching offices, junction pairs, both. "Links" represent cable, terminating equipment, etc. "Traffic" is information within the network that flows through the nodes and links. With the introduction of more sophisticated services involving the transfer of large amounts of information, such as digital data, video, or voice data, communications networks must provide certain levels of performance.
As the speed of serial data links and communications networks increase, the quality of the data must be maintained. For example, jitter and signal amplitude may cause intersymbol interference (ISI). "Jitter" is a term which refers to a type of signal distortion in serial data communications. More specifically, jitter can be described as incidental phase modulation measured as a time deviation. Typical bit error rates (BERs) are acceptable to an order of 10.sup.-12. Transmitters with low jitter output and sensitive receivers are required in serial data links to achieve acceptable levels of BER in communications networks. Additionally, repeaters are used to regenerate the amplitude with the signal providing the ability to transmit the data across the additional lengths of cable, allowing quality transmission over longer distances. Additionally, repeaters are employed to remove jitter caused by ISI. A component found in many repeaters is a phase locked loop (PLL) circuit, which is used to provide a recovered clock signal to re-time data going through a repeater. Many phase locked loop circuits employ a reference clock as an input to the repeater. The reference clock usually consist of an expensive crystal oscillator, which is used only for a short period of time to obtain frequency lock within the clock recovery PLL. Once frequency lock is obtained, the reference clock is no longer used by the repeater circuit.
In FIG. 1, repeater circuit 100 includes an amplifier 102, a D flip-flop 104, a clock recovery PLL 106 and a reference clock 108. Data is input into amplifier 102, which regenerates the amplitude of the data signal. This data is sent to D flip-flop 104 and clock recovery PLL 106. Reference clock 108 is employed to obtain frequency lock when the circuit is started. Thereafter, clock recovery PLL 106 generates a recovered clock signal, which is input into D flip-flop 104 as a clock signal that re-times data output from D flip-flop 104.
In reducing costs of PLLs and repeaters, it would be desirable to have an improved clock recovery apparatus that eliminates the requirements of a reference clock using a crystal oscillator.